Along with technical development and increasing demand for mobile devices, demand for secondary batteries as an energy source is rapidly increasing. In particular, currently lithium secondary batteries having high energy density, high working voltage, long cycle lifetime, and low self-discharge rate are widely commercially available.
Such a lithium secondary battery has a structure with an electrolyte assembly impregnated with an electrolyte solution containing a lithium salt, the electrolyte assembly including a positive electrode and a negative electrode that are obtained by coating electrode current collectors with respective positive and negative active materials and are separated from one another by a porous separator. During charging, lithium ions released from the positive active material are intercalated into a negative active material layer. During discharging, the lithium ions released from the negative active material layer are intercalated into the positive active material. The electrolyte solution serves as a migration medium of lithium ions between the negative electrode and the positive electrode.
In general, an electrolyte solution may include an organic solvent and an electrolyte salt. For example, a widely used electrolyte solution may consist of a mixed solvent of a high-dielectric cyclic carbonate such as propylene carbonate, or ethylene carbonate, and a low-viscosity chain carbonate such as diethylcarbonate, ethylmethylcarbonate, or dimethylcarbonate, and a lithium salt such as LiPF6, LiBF4, LiClO4 added to the mixed solution.
Lithium-containing halide salts such as a lithium-containing fluoride salt or a lithium-containing chloride salt which may be used as the electrolyte salt are highly sensitive to moisture, generating a strong acid (HX, wherein X is F, Cl, Br, or I) by reaction with moisture during the manufacture of a battery or with moisture present in the battery. In particular, since LiPF6 as a lithium salt is unstable at high temperature, its anions may be thermally decomposed, generating an acidic material such as hydrofluoric acid (HF). This acidic material may unavoidably accompany an undesirable side reaction within the battery.
For example, a solid electrolyte interphase (SEI) layer on a surface of the negative electrode may be vulnerable to damage due to strong reactivity of the hydrofluoric acid (HF), which may induce continuous regeneration of the SEI layer, and increase the thickness of the coated SEI layer of the negative electrode and an interfacial resistance of the negative electrode. As lithium fluoride (LiF) as a byproduct of the generation of hydrofluoric acid (HF) is adsorbed onto the surface of the positive electrode, an interfacial resistance of the positive electrode may also be increased. In addition, the strong acid (HX) may cause radical oxidation reaction within the battery, and consequently dissolution and degeneration of the electrode active materials. In particular, as transition metal cations included in a lithium metal oxide used as a positive active material are dissolved, the cations may be electrodeposited onto the negative electrode, forming an additional coating layer on the negative electrode, consequently further increasing the resistance of the negative electrode.
The SEI layer, which may be formed on the surface of the negative electrode by reaction of a polar non-aqueous carbonate solvent with lithium ions of the electrolyte solution during initial charging of a lithium secondary battery, may serve as a protective layer by inhibiting decomposition of the carbonate electrolyte solution to stabilize the battery. However, an SEI layer formed of only an organic solvent and a lithium salt may not be enough to consistently serve as a protective layer and thus may be gradually damaged during continuous charging and discharging of the battery or storage of the battery at high temperature in a fully charged state, due to increased electrochemical energy and heat energy. A side reaction of decomposing a surface of the negative active material exposed through the damaged SEI layer by reaction with the electrolyte solution solvent may continuously occur, leading to deterioration in characteristics of the battery, including capacity reduction, lifetime reduction, and resistance increase. Such a side reaction may generate gas in the battery. As such gas generation is continued, the internal pressure of the lithium secondary battery may be increased at high temperature, causing swelling of the battery with increased thickness and finally raising a safety concern of the battery.
To address these drawbacks, Patent document 1 (JP 2002-313415) discloses a non-aqueous electrolyte solution including about 0.5 wt % to about 1.5 wt % of biphenyl and 0.5 wt % to about 2.0 wt % of cyclohexyl benzene (CHB) as additives. According to this disclosure, swelling of a battery in a thickness direction may be reduced when the disclosed battery is left even at high temperature. A high-temperature characteristic test performed by measuring a thickness of the battery after it was maintained at a high temperature for 2 days supports that swelling of the battery in the thickness direction was slightly suppressed, but not any battery characteristic improvement effect. In particular, when the storage period at high temperature is continued for a longer time, battery capacity and capacity retention may be markedly reduced even with using the additives.